Method of and device for gear teeth hardening

ABSTRACT

The method provides for gear teeth plastic strain by curving them in the direction of application of the principal working load. The device for carrying the method into effect comprises a bed which carries a ring-shaped mechanism for setting up plastic strain of the teeth of the gear wheel being treated, and a clamping fixture for fitting the gear wheel into the ring and for turning the gear wheel with respect to the latter. The ring inner surface is provided with tooth-like projections adapted to interact with the gear teeth when the gear wheel rotates in the ring so as to curve the teeth.

[ Dec. 3, 1974 METHOD OF AND DEVICE FOR GEAR 2,206,831 7/1940Bcrthelsen......................... 2,860,077 11/1958 Adairet 211......3,466,909 9/1969 TEETH HARDENING [76] Inventors: German AlexandrovichZhuravlev,

ulitsa Verkhne-Nolnaya, l0, kv.9; Efim Grigorievich Roslivker,

Primary Examiner-Lowell A. Larson Attorney, Agent, or Firm-Holman &Stern prospekt Budennovsky, 72, kv.5 l; Eduard Mikhailovich Chilevich,

pereulok Solyanoi Spusk, 6, kv.37; Vladimir Samuilovich Gurevich,

ABSTRACT prospekt Oktyabrya, 18, kv.20, all of Rostov-na-Donu, U.S.S.R.

Mar. 8, 1973 The method provides for gear teeth plastic strain by 22 Flcurving them in the direction of application of the I 1 1e principalworking load. The device for carrying the method into effect comprises abed which carries a ring-shaped mechanism for setting up plastic strainof [21] Appl. No.: 339,349

the teeth of the gear wheel being treated, and a clamping fixture forfitting the gear wheel into the ring and for turning the gear wheel withrespect to the latter. The ring inner surface is provided withtooth'like projections adapted to interact with the gear teeth when thegear wheel rotates in the ring so as to curve the teeth.

1,444,259 Lothrop.............................. 148/131 9 Claims, 5Drawing Figures PATENTEL DEC 3 I974 SHEEI 1 0F 4 PATENTLL EB 1974SHEUEUF 4 F/EIZ MENIL ESE 31974 SHEET 30F 4 v METHOD OF AND DEVICE FORGEAR TEETH HARDENING BACKGROUND OF THE INVENTION This invention relatesgenerally to the sphere of hardening technique of machine parts by wayof modifying the physical properties of metal due to plastic strain, andmore specifically, it relates to a method of and a device for gear teethhardening.

The invention can find most utility when employed for hardening theteeth of gear wheels upon which unidirectional load is normally imposed.

In the present-day practice, there are widespread applications formethods of improving the strength and hardness of machine parts andelements, by using plastic strain of the metal of a part set up duringthe process of hardening.

As a rule, plastic strain of metal in such cases is not aimed at shapingof the parts but is confined only to modifying the physical propertiesof metal. Referred to physical properties of metal that experiencealternations in the course of such a treatment, should be the limit ofelasticity, ultimate strength, hardness, macrostructure and internalstresses.

As it is known commonly, plastic strain of metal that is capable ofbeing hardened, results in an increased limit of elasticity, ultimatestrength and hardness of the material as compared to the initial valuesof such.

Moreover, if any cross section of the part should be in non-uniformlystressed state in the course of plastic or elasto-plastic strain, thisresults in residual stresses that will appear in the cross section uponrelieving the applied load due to different degrees of strain on variouscross-section areas.

Pertaining to the most widespread metal hardening technique that usesthe physical phenomena, is the plastic strain of the surface of a partreferred to as surface peening. In particular, it is widely known to usesome gear teeth hardening methods by virtue of plastic strain of thesurface of teeth metal. Such a plastic strain is attained due toshot-peening of the surface to be hardened or bumishing with rolls withthe result that the strength characteristics of the teeth surface layerare improved and that some residual stressesare left in the layerarranged so as to favorably increase the loadbearing ability of theteeth as a whole.

In one of the prior-art methods, such a hardening is attained due tosurface cold peening of gear teeth by virtue of plastic strain of metalalong the entire tooth profile which occurs during the bumishing thereofwith toothed rolls.

In such case, the plastic strain of metal is due to a preliminary radialdisplacement of the toothed rolls which are set with a definiteinterference corresponding to the required degree of hardening. Whilemeshing the gear teeth being hardened, the bumishing roll teethplastically strain the metal of the latter, thus effecting metal surfacecold peening which improves its strength characteristics.

Gear teeth hardening by virtue of plastic strain is carried out, forinstance, by the device according to USSR Inventors certificate US. Pat.No. 158,911, Class 18c, 1/80 (Int. C1. (21), comprising a drive and abed carrying a mechanism to set up plastic strain, and an arrangementfor fixing the gear wheel being hardened.

The mechanism for setting up plastic strain is made as a number ofbumishing toothed rolls located in a common housing, mounted oneccentric pivots and imparted vertical motion along with the housing andradial motion due to the turning of the eccentric pivots.

Gear teeth hardening occurs due to an interference established betweenthese and the teeth of the burnishing rolls.

In this known method, the effect of the teeth metal hardening is moresuperficial in nature is spread but to a limited depth, e.g., maximum 3mm for the teeth of gears having a module of 10-1 1 mm. This results inthe fact that the depth of residual compressive stresses is not inexcess of 0.7 mm for the given particular case, on account of which thestresses exercise but insignificant influence upon the distribution ofstresses in gear teeth under working load.

The bumishing rolls of the plastic strain setting-up mechanism of theknown device, are capable of straining only a thin superficial layer ofthe teeth material which eventuates in that an improvement in thematerial strength characteristics and residual compressive stresses inthe hardened layer result but also results in an inconsiderable increaseof the teeth bending strength as a whole.

An attempt to increase the hardened layer at the expense of a higherinterference between depth of the gear teeth being hardened and theteeth of the burnishing rolls failed to yield the desired effect sinceit only resulted in metal flaking due to its surface overhardenmg.

It should be pointed out that the depth of penetration and character ofdistribution of residual compressive stresses exert a decided influenceupon the bending strength of gear teeth under treatment.

However, the now existing hardening methods fail to set up such adistribution of stresses at which stresses in the teeth material underworking load would be distributed adequately over the entire criticalcross section.

Th distribution nature of residual stresses with the use of the knownhardening methods leads to the fact that maximum stresses from theeffect or working load in the tooth critical cross-section occur in thesubsurface layer, while the core material participates butinconsiderably, in taking-up the working load, thus practically escapingfrom the working process.

SUMMARY OF THE INVENTION It is therefore an essential object of thepresent invention to provide a method of and a device for gear teethhardening that will enable a volumetric cold peening of teeth materialin the critical tooth cross-section to increase hardening depth, as wellas the value and depth of penetration of residual compressive stressesso that all this will result in a considerable increase in the bendingstrength of gear teeth.

This and other objects are attained in a method of gear teeth hardeningby virtue of plastic strain, wherein, according to the invention theplastic strain is set up by curving the gear teeth in the direction ofapplication of the principle working load.

Due to the fact that the teeth of the gear being treated are subjectedto curving which causes crippling stresses in the tooth material havingthe same sign as the working load stresses, high residual stresses aredeveloped penetrating for a considerable depth in the teeth criticalcross-section, the stresses exercising a substantial influence uponredistribution of stresses under working load, thus ensuring in the gearteeth working process a more favorable energetic state of the teethmaterial over the entire critical cross-section, the core inclusive.

Furthermore, plastic bending strain makes it possible to effect coldpeening of the teeth material on the majorportion of the criticalcross-section which is unattainable by any of the known hardeningmethods.

It is expedient that prior to curving the teeth their contact surfacesbe subjected to such a heat treatment after which the total surfacehardness of every tooth within the zone of adjacency of the hardenedlayer and the cold-peened layer would vary adequately along the toothheight.

Such an embodiment makes it possible to ensure high contact strength ofgear teeth and at the same time obviate any additional concentration ofstresses which result due to the absence of hardness difference withinor between adjacent zones of the hardened and the coldpeened layers.

It is likewise reasonable that the teeth surface be subjected, eithersimultaneously with curving (bending) or thereupon, to heating followedby cooling in the loaded state.

Such an embodiment makes it possible to attain the following:

a. in case of heat treatment carried out simultaneously with curving, toensure high contact strentth of gear teeth made of carbon orcase-hardenable steels;

b. to harden by the present method, the gear teeth made of carbon orcase-hardenable steels that have passed deep hardening to high degree ofhardness or a complete thermochemical treatment due to a degradedelasticity limit of the material within the teeth fillets occurring inthe course of heating during heat treatment procedure;

c. in case of heat treatment occurring after teeth curving (to suit thetechnological requirements of a particular production practice), toensure high contact strength and increase bending strength due tothermomechanical treatment of-the teeth fillet zone of those gears whosematerial is not subject to heat treatment after tooth milling process.

A device for carrying into effect the method of gear teeth hardening byvirtue of plastic strain effected by curving or bending the teeth in thedirection of application ofthe principle or primary working load, whichdevice comprises a bed which carries a mechanism to set up plasticstrain, a clamping fixture for the gear wheel being treated is secured,and a drive to impart relative motion to the mechanism and the gearwheel.

According to the invention, the mechanism for setting up plastic straincomprises a ring having on its inner surface at least one tooth-likeprojection which is adapted to successively interact with the teeth ofthe gear wheel along the entire length thereof in the course of therelative movements of the ring and the gear wheel set concentricallytherewith on the fixture, thus curving said teeth.

Such a constructional feature makes it possible to apply to the teeth ofthe gear wheel under treatment a bending moment (hereinafter referred toas technological load) that sets up in the gear teeth material stressesbeyond the yield point thereof and having the same sign (direction) asthe working load.

It is expedient that each tooth-like projection be made as an individualstraining element which is located in respective longitudinal slotprovided in the ring inner surface and be free to effect movement squarewith the gear teeth surface being treated.

Such a constructional feature makes it possible to degrade the accuracyrequirements of the mechanism for setting up or imposing plastic strainand to compensate for geometrical errors of the gear wheel undertreatment due to set-up motion of the straining elements.

In case of treating gear wheels having longitudinally curvilinear teeth,the straining elements may be builtup in length, i.e., composed of atleast two separate portions establishing a curvilinear surface.

Such a constructional feature makes it possible to uniformly spreadtechnological load lengthwise the tooth which is hardly attainable withthe use of a solid straining element.

In order to also impart to the mechanism for setting up plastic strainthe functions of an inductor capable of heating the gear wheel beingtreated, the ring of the mechanism has an open-end radial groove.wherein a gasket is placed, while split ring ends are interconnectedthrough current-leading bus-bars to a H.F. (high-frequency) currentsource; the ring inner surface is shaped to cooperate with the shape ofthe gear teeth being treated.

Such a constructional feature makes it possible, alongside with teethcurving or thereupon, to subject the teeth surface to heating followedby cooling when the teeth are in the strained state.

The current-carrying bus-bars can be made integral with the ring and beused for hanging or supporting the latter; it is also expedient toprovide movable clamps located in a groove on the bed of the device andwhich clamps are adapted to fix the ring on the bed at the moment ofapplication of the load setting up plastic strain.

To promote a better understanding of the essence of the presentinvention, specific embodiments of a method of gear teeth hardeningcarrying into effect by a device represented in the accompanyingdrawings are described in detail below, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective,fragmentarily cutaway view of an embodiment of the device for gear teethhardening using the novel method;

FIG. 2 is an enlarged sectional view taken along the line IIII in FIG.1;

FIG. 3 is a developed view of the slot in the mechanism for setting upor imposing plastic strain, in case of treating curvilinear tooth gears;

FIG. 4 illustrates the diagrams of normal stresses effective at a toothroot and resulting from:

A the effect of bending (technological) load;

B the effect of residual stresses;

C the effect of permissible working load for a tooth hardened by theproposed method;

C the effect of permissible load for a nonhardened tooth;

D the effect of resulting stresses; and

FIG. 5 shows another embodiment of the device for gear teeth hardeningaccording to the novel method with the use of heat treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now referring to FIG.1, the device according to the invention comprises a bed 1 which carriesa mechanism 2 for setting up plastic strain; a clamping fixture 3 tosecure a gear wheel 4 under treatment; and a hydraulic drive 5 forturning the gear wheel 4 with respect to the mechanism 2.

The bed 1 is essentially a metal plate strong enough to sustaintechnological load.

The mechanism 2 for setting up plastic strain is made as a cylindricalring 6 which is fixed in vertical position on the bed 1 by means of astand 7.

The ring 6 has on its inner surface a number of toothlike projectionsmade as individual detachable straining elements 8 fitted in respectivelongitudinal slots 9 (FIG. 2) which are equi-distantly spaced along theperiphery of the ring 6.

The slots 9 are rectangular in cross section, while their spacing ismultiple of the pitch of teeth 10 of the gear wheel 4 being treated.

A rear wall 11 of each slot 9 is somewhat inclined, whereas eachstraining element 8 is respectively wedgeshaped in longitudinal sectionfor its setup motion square with the surface of the teeth 10 of the gearwheel 4 being treated.

A surface 12 of the straining element 8 that is adapted to interact withthe tooth 10 of the gear wheel 4, is convex-shaped to ensure a linearcontact with the surface of the tooth 10 being hardened.

In case the teeth 10 being hardened are of the straight involute typethe surface 12 is cylindrical in shape.

In cases where the teeth 10 of the gear wheel 4 being treated arecurvilinear in longitudinal direction, it is oftentime difficult to makethe solid straining elemnt 8 which can ensure a linear contact ensuringa uniform load distribution over the entire length of the tooth 10.

In such cases each straining element may be made built-up in length andcomposed of several individual parts 13 (FIG. 3), whose working surfaces14 form a curvilinear surface adapted to be in contact with the toothsurface being hardened.

The clamping fixture 3 (FIG. 1) to secure the gear wheel 4 being treatedcomprises two stands 15 connected fixedly on the bed 1 at both butt endsof the ring 6 of the mechanism 1 and carrying a horizontal splined shaft16 mounted in the stands on radial bearings 17 concentrically with thering 6. The shaft 16 is to mount the gear wheel 4 being treated.

The extensions of the shaft 16 protrude from the stands 15 carrysingle-arm levers l8 interconnected by a bar (shaft) 19 which by meansof a pitman 20 is, in turn, articulated to a rod 21 of a cylinder 22 ofa hydraulic drive 5 which is adapted to impart rotation to the shaft 16along with the gear wheel 4 set thereon, with respect to the ring 6.Fluid lines are piping 23 communicate the cylinder 22 with the oilpumping stations (not shown) operating the hydraulic drive 5.

The proposed device may also be implemented according to an alternativeembodiment, wherein the gear wheel 4 being treated is stationary, whilethe relative motion is performed by virtue of the mechanism for settingup or effecting the plastic strain.

The afore-described device operates as follows.

The gear wheel 4 being treated is mounted on the splined shaft 16 havingextensions in the bearings 17 fitted in corresponding holes in thestands 15 so that the gear wheel 4 is located inside the ring 6 0f themechanism 2 for imposing strain.

Thereupon, the straining elements 8 are installed into the slots 9 ofthe ring 6 at one of the butt ends thereof and are imparted setupmovements perpendicular to the surfaces being hardened of the respectiveteeth 10 of the gear wheel 4 so as to uniformly distribute technological(bending) load between all the simultaneously hardened teeth 10.

Next, the hydraulic drive 5 is put into operation with f the result thatthe connecting rod 21 of the hydraulic cylinder 22, while acting throughthe pitman 20, moves the shaft or bar 19 which turns the levers 18 atthe angle a in the direction of the arrow E (FIG. 1); (the directionshould coincide with the direction of application of the principalworking load imposed on the gear wheel 4) and along with the levers alsothe shaft 16 with the gear wheel 4 mounted thereon rotates with respectto the ring 6.

In the course of the rotation, the straining elements 8 of the ring 6interact with the teeth 10 of the gear wheel 4, thus setting up orimposing plastic strain of the teeth 10 by curving these in thedirection of application of the principle (primary) working load, withthe result that the material of the teeth 10 is given bulk peeningwithin the zone of critical cross section.

The angle a at which the gear wheel 4 turns with respect to the ring 6is estimated or determined proceeding from the geometrical parameters ofthe gear wheel 4 being treated and the characteristics of the materialof its teeth 10. Thus, for a spurgear with a module of 10 mm andnumbering of 17 teeth made of steel alloyed with up to 1 percent ofchromium and 0.4 percent of carbon and hardened to 38 l-IRC, the angleis equal to 025.

Once the splined shaft 16 has turned at the angle a, the device isautomatically disengaged, the straining elements 8 participating in thehardening process are brought out of contact with the thus-treated teeth10 of the gear wheel 4; then another group of the straining elements 8is given setup motion, and the whole cycle is repeated.

The number of the teeth 10 that are subject to treatment at one time isdetermined proceeding from constructional reasons, depending upon thestrength of the components of the proposed device.

As a result of treatment of gear wheels by the proposed method thematerial of their teeth is subjected to bulk peening for a depth h (asshown in the diagram A in FIG. 4) of the cross section. Besides,residual stresses arise in the thus-hardened tooth (see the diagram B inResidual stresses resulting from machining are distributed over thetooth cross section in such a manner as to exercise a considerablefavorable influence upon the redistribution of stresses from the effectof the working load (see the diagram D in FIG. 4), which results in anincrease of the permissible fatigue stress with respect to bending fromto [ti- (see the diagrams C and C in FIG. 4).

Tabulated below are the characteristics of gear ent of total hardnesswithin the zone of tooth fillet equals I HRC/mm.

Such a treatment prevents the appearance of additional stressconcentrations which are liable to occur in wheels treated by the knownmethod versus those after the zone of emergency of the end of a layerthat has treatment by the proposed method for increasing teeth eInduction hardened, due to an abrupt hardness bending strength (seeTable 1). difference; I

The table contains data on gear wheels having a mod- The device shownll] 5 is adapted fOf l'8lilZZ1tlOl'l ule of 10 mm made ofcase-hardenable steel alloyed 0f the P P method in eases Where eitherPrior to with up to 1 percent of the chromium and 0.2 percent 10 vtootheurvmg e thereupon. the teeth 6 1S f of carbon, that have been hardenedby the proposed Jeeted t0 heatmg followed y eoohhg the Strthhed method,whereas data on the known method refer to Stategear wheels made of steelcontaining up to 1 percent of The tievlce llke fi ISt descrlbed aboveand liiUS- Chromium and 4 percent f carbon and hardened up trated inFIG. 1, comprises a bed 24 which carries :1 to HB 300 mechanism 25 forsetting up plastic strain, a clamping An increase in load-bearingability is expressed in the fixture for seeuhhg the gear Wheel 4 beingtreated ratio and a drive (not shown) for the gear wheel 4 to turn withrespect to the mechanism 25.

The mechanism 25 is made as an inductor which is lnar] yl[ mu.r]H

. a H W essentially a cylindrical ring 27 whose inner surface is made tosuit the shape of the teeth 10 of the gear wheel which equals toapproximately 2.6-2.8. 4 being treated.

Departure e the true e e l shepe of the The rings 27 has an open-endradial slot, wherein an teeth profile ansmg f p astle e eempen'insulating gasket 28 is placed, whereas the split ends of sated ether bya prehmmary alterenon of the teeth the ring 27 are made integral withcurrent-carrying bus- Shape or by the Subsequent machmmg of the Surfacebars 29 separated by the gasket 28 and communicating thereof the ringwith the source of HF current through termiln some cases a satisfactorydegree of accuracy may nals L and beattaiiied due to an appropriateselection of the hard- The ring 27 is Suspended through the bus barsemhg Parameters 30 Mounted in a groove 30 on the bed 24 are movable 1heases WheTe subleeted t0 hardening by the P clamps 31 adapted to gripthe ring 27 and thus fix it on Posed methed are teeth through wheseContact the bed 24 at the moment of application of technologifaces arepassed HF induction hardening currents, the cal load. latter isPreferable t Occur P130r to teeth eurvmg; 1h Mounted on the innersurface of the ring 27 in ret eesethe e of the hardhess 9 the Surfacespective longitudinal slots are straining elements 32 being hatdehedWlthlh the Q t edlaeehey to the which are similar to the strainingelement 8 described layer helhg eold'peehed Wlthlh the Zone of h withreference to the device illustrated in FIG. 1. tooth fillet) must be sopreset that the total hardness in passageways 33 are provided in thebulk f the ring the zone after tooth curving will vary uniformly for the27 f cooling ]iquid tooth height- The clamping fixture 26 for securingthe gear wheel The P t law of Variant?" of e hardness of a layer 4 beingtreated in the inductor ring 27 comprises stands hardened wlt 'lFCurrent, 1 5eX8f1S ed y known meth- 34 mounted on the bed and carrying asplined shaft 35 ods by appropriately selecting theinductor shape and hih i d d f Setting h gear h l 4 b i the frequency of t e supply Currenttreated coaxially with the ring 27.

For gear teeth having a module of 10 mm Whose The extensions of theshaft 35 carry levers 36 interterial has an lmtlal hardn of 38 and theconnected through a bar 37 which in turn is associated contact surfacesthat have been induction hardened, i h h d i h feature a Rockwellhardness of HRC=4 850, the g The above-described device operates asfollows.

Table l Nos. Description of parameter Designa- Unit of Parameter valuetion measurenon-harcase-hardenhardened harment dened ed or hardenbyknown dcned teeth ed over entimethod by (marre tooth proposked H) spaceed methat! (marked y) 1. Depth of hardened layer h mm 2.0 2.29 8.3 2.Depth of penetration of residual compressive stresses 8 mm 2.7 0.65 3.43. Maximum value of residual compressive stresses 0' kgf/cm 2400 96005600 4. Permissible fatigue stress of material at point of zero residualstresses (1,] kgf/cm 2600 2600 3600 3600 5. Permissible fatigue stressagainst surface fatigue fracture [i7,.,,,] kgf/cm 2600 3500 4900 7700 6.Degree of increase in breaking [cr,,,,, /[a,,,,,,],, l 1.35 1.90 2.80

strength Similarly to the afore-described process true for the device inFIG. 1, the gear wheel 4 being treated is set on the shaft 35 so as toaccommodate it in the ring 27, and the straining elements 32 areappropriately set up, whereupon by turning the levers 36 the teeth 10 ofthe gear wheel 4 are withdrawn from the straining elements 32 for fearof a short circuit.

Then the HF hardening arrangement is put into operation, and a high-ratesurface heating of the teeth 10 of the gear wheel 4 occurs. Upon heatingthe surface of the teeth 10 to a temperature exceeding the hardening oneby 150250C, the HF hardening arrangement is turned out (for fear ofbreakdown and partial melting of the teeth) and the ring 27 is grippedby means of the clamps 31 movable from the hydraulic drive along thegroove 30 of the bed 24 in the direction of the arrow P. The samehydraulic drive then causes the levers 36 to move in the direction ofthe arrow Q, thus effecting a turn of the gear wheel 4 with respect tothe ring 27.

case of elastoplastic strain of the teeth due to their curving withoutinduced heating.

In case of using heat treatment of gear teeth with the employment of thedevice shown in FIG. 5, upon sub- 5 jecting the teeth to elastoplasticbending (in keeping In the course of turning the straining elements 32interact with the teeth 10 of the gear wheel 4, thus imparting plasticstrain to the teeth 10 by curving these in the direction of applicationof the principal working load.

During the curving process, the surface of the teeth 10 of the gearwheel 4 being treated are spray-cooled with the aid of a sprinklernozzle (not shown) located at butt ends of the ring 27.

As one of the cycles of hardening a group of teeth by the methodaccording to the invention is finished, a next group of the teeth 10 ofthe gear wheel 4 must proceed to be hardened.

Upon completing the treatment, the hardened gear wheels may be subjectedto low-temperature temper.

The angle of relative turn of the gear wheel 4 in the course oftreatment, for instance, for gears having a module of 10 mm and thenumber of teeth 17 being equals 10-20".

In cases where the proposed method is used for treating the gear teethmade of carbon steels, or of casehardenable steels that have passedchemical treatment without hardening, the whole tooth space of the gearteeth being hardened is subjected to surface heating.

Thus, there results high hardness of teeth contact surfaces that ensureshigh contact strength thereof and, at the same time, an additionalincrease in the bending stength at the expense of thermomechanicaltreatment of the material superficial layers within the zone of thetooth fillet. The frequency of HF current for such a treatment is 250kHz, the heating time being 1-2 sec.

In those cases where subjected to hardening are gear teeth that havepreliminarily passed heat treatment to high degree of hardness, or acomplete thermochemical treatment, the surface heating is applied onlyto the zone of the teeth fillets, which is attained due to anappropriate selection of the inductor supply current frequency. Currentfrequency in such cases falls within the range of 2.5-8 kHz and heatingtime equals 23 sec.

As a result of heating the elasticity limit of the material is more thantwice as low in the layers subject to maximum strain load, which makesit possible to obtain plastic deformation of cold material of the teethcore and establish residual stresses of the same nature as in with therequirements of every particular production technology), the proposedmethod proceeds with the heating of the surface of the whole tooth spacewhich ensures high contact strength of the teeth being treated due totheir thermal hardening, as well as an increased bending strength at theexpense of elastoplastic deformation of the tooth core andthermomechanical treatment of the superficial layer within the zone ofthe tooth fillet.

The specific power consumption required for the realization of theproposed method, equals O.9-l .5 kW/cm What we claim is:

l. A method of gear teeth hardening by bulk peening comprising incombination the steps of:

A. providing a toothed gear element having a principal working surfacewhich normally receives the principal working load when the gear isoperative; and a B. applying a uniform technological load across andalong selected areas of the principal working surface of the teeth ofthe gear element and strengthening the teeth along their length bybending the teeth and bulk peening of the principal working surfacewithin a zone of critical cross section in the direction of applicationof principal working load.

2. A method as claimed in claim 1, including the step of heating contactsurfaces of the gear teeth prior to applying said technological load sothat the total surface hardness of every tooth within a zone ofadjacency of a hardened layer and a cold-peened layer varies uniformlyalong the tooth height.

3. A method as claimed in claim 1 comprising the successive steps of:heating the surface of the gear teeth; applying the technological loadand subsequently cooling while the teeth are in a strained state.

4. The method as claimed in claim] comprising the successive steps of:applying the technical logical load to the teeth; heating the teethsurface and subsequent cooling while the teeth are in a strained state.

5. A device for carrying into effect a method of gear teeth hardening,comprising: a bed; a mechanism capable of establishing plastic strainand mounted on said bed, said mechanism being made as a ring whichaccommodates a gear wheel under treatment, said gear wheel being setconcentrically with said ring; means for clamping the gear wheel beingtreated inside said ring; drive means for imparting relative motion ofsaid mechanism and said gear wheel under treatment; said ring of saidmechanism having at least one tooth-like projection disposed on itsinner surface, which is adapted in the course of relative movements ofsaid ring and the gear wheel being treated, to successively interactwith the teeth of said gear wheel along the length of the teeth, thuscurving the latter.

6. A device as claimed in claim 5, wherein each said tooth-likeprojection is made as an individual straining element disposed in arespective longitudinal slot in the inner surface of said ring and iscapable of performing set-up motion square with the gear teeth surfacebeing hardened.

an inductor; the inner surface of said ring being shaped to suit theshape of the teeth of said gear wheel being treated.

9. A device as claimed in claim 8, wherein said current-carryingbus-bars are made integral with said ring and serve for hanging thelatter, while said bed of the device carries movable clamps mounted in agroove and adapted to fix said ring at the moment of application of theload setting up plastic strain.

1. A method of gear teeth hardening by bulk peening comprising in combination the steps of: A. providing a toothed gear element having a principal working surface which normally receives the principal working load when the gear is operative; and B. applying a uniform technological load across and along selected areas of the principal working surface of the teeth of the gear element and strengthening the teeth along their length by bending the teeth and bulk peening of the principal working surface within a zone of critical cross section in the direction of application of principal working load.
 2. A method as claimed in claim 1, including the step of heating contact surfaces of the gear teeth prior to applying said technological load so that the total surface hardness of every tooth within a zone of adjacency of a hardened layer and a cold-peened layer varies uniformly along the tooth height.
 3. A method as claimed in claim 1 comprising the successive steps of: heating the surface of the gear teeth; applying the technological load and subsequently cooling while the teeth are in a strained state.
 4. The method as claimed in claim 1 comprising the successive steps of: applying the technical logical load to the teeth; heating the teeth surface and subsequent cooling while the teeth are in a strained state.
 5. A device for carrying into effect a method of gear teeth hardening, comprising: a bed; a mechanism capable of establishing plastic strain and mounted on said bed, said mechanism being made as a ring which accommodates a gear wheel under treatment, said gear wheel being set concentrically with said ring; means for clamping the gear wheel being treated inside said ring; drive means for imparting relative motion of said mechanism and said gear wheel under treatment; said ring of said mechanism having at least one tooth-like projection disposed on its inner surface, which is adapted in the course of relative movements of said ring and the gear wheel being treated, to successively interact with the teeth of said gear wheel along the length of the teeth, thus curving the latter.
 6. A device as claimed in claim 5, wherein each said tooth-like projection is made as an individual straining element disposed in a respective longitudinal slot in the inner surface of said ring and is capable of performing set-up motion square with the gear teeth surface being hardened.
 7. A device as claimed in claim 6, wherein said straining element is built-up in length for treating said gear wheel having longitudinally curvilinear teeth, the length being composed of at least two individual portions that establish a curvilinear surface.
 8. A device as claimed in claim 5, wherein said ring has a split end and is profided with a radial groove with an insulating gasket placed therein, while the split ends of said ring are inter-connected through current-carrying bus-bars with a H.F. current-source, all this being provided for said ring to perform the function of an inductor; the inner surface of said ring being shaped to suit the shape of the teeth of said gear wheel being treated.
 9. A device as claimed in claim 8, wherein said current-carrying bus-bars are made integral with said ring and serve for hanging the latter, while said bed of the device carries movable clamps mounted in a groove and adapted to fix said ring at the moment of application of the load setting up plastic strain. 